1. Field of the Invention
The present invention relates to a method of driving a simple matrix type liquid crystal display panel using STN liquid crystals or the like and, more particularly, to a method of driving a liquid crystal display panel so that time information is displayed on a portion of the screen when the display is not used for a mobile phone function or the like.
2. Description of the Related Art
A simple matrix type liquid crystal panel is constituted by maintaining a liquid crystal layer between a row electrode group and a column electrode group to define a plurality of pixels in matrix form. Further, as methods for driving the simple matrix type liquid crystal display panel, there are the voltage averaging drive method, the SA drive method, and the MLA drive method.
The voltage averaging drive method is a method of driving a simple matrix type liquid crystal display panel for successively selecting respective row electrodes piece by piece and providing all the column electrodes with data signals in correspondence with ON and OFF in accordance with selected timings. Therefore, a voltage applied to respective electrodes becomes high only once in one frame cycle T for selecting all the row electrodes and becomes constant bias voltage during a remaining nonselection time period. According to the voltage averaging drive, when a response speed of the liquid crystal material used is slow, there is provided a change in brightness in accordance with the effective value of the waveform of the applied voltage in the one frame cycle to thereby maintain the most suitable contrast for the conditions. However, when the division number is increased and frame frequency is reduced, the difference between frame cycle time and response time of liquid crystal is reduced, the liquid crystal responds separately to each applied pulse, there appears flicker of brightness referred to as a frame response phenomenon, and the contrast is reduced.
The SA drive method is a method of driving a simple matrix type liquid crystal display panel and is referred to as a smart addressing method. The voltage averaging drive and the SA drive methods both select each row of electrodes one row at a time in order, and provide a data signal corresponding to turn on or turn off to each column of electrodes at a selected timing. However, common nonselection levels for adjacent frames differ in the voltage averaging drive, but are the same in the SA drive. Therefore, in the SA drive, too, when the division number is increased and frame frequency is reduced, there appears flicker of brightness referred to as a frame response phenomenon and the contrast is reduced similar to the voltage averaging drive. Further, a common driver waveform voltage is applied to a positive polarity side and a negative polarity side centered about a nonselect level. Consequently, an IC withstand voltage of twice that used in the voltage averaging drive becomes necessary.
The MLA drive method is also referred to as the multiple line addressing or multiple line selecting method for simultaneously selecting a plurality of row electrodes so that apparent high frame frequency formation is achieved and the frame response phenomenon which is problematic in the voltage averaging drive and the SA drive methods is restrained. The MLA driving method simultaneously selects a plurality of row electrodes and display respective pixels independently from each other. In this scheme there is carried out set successive scanning applying a plurality of row signals represented by a set of orthogonal functions to a row electrode group according to a set order for each respective selection time, there is successively carried out a crossproducts operation between the set of orthogonal functions and a set of selected pixel data, and column signals having voltage levels in accordance with the result of the operation are applied to a column electrode group during the selection time in synchronism with the successive scanning of the set (See JP 06-236167 A).
As described above, the SA drive has disadvantages in that the frame response phenomenon appears, and driver voltages become higher, when the number of divisions becomes larger. However, the frame response phenomenon does not appear when the number of divisions is small, and the driver voltage can also be reduced. The electric power consumption can also be made smaller compared to the MLA drive because there is no product and summing operations.
Further, the MLA drive has a disadvantage in that product and summing operations are performed. The number of times where data is read out from memory is thus increased, and the electric power consumption in a logic circuit portion becomes higher than that of the SA drive. However, the MLA drive has an advantage in that there is no frame response phenomenon, even if the number of divisions is large.
Voltage levels necessary for signal electrodes and scanning electrodes for each method are examined next. FIG. 2 shows a voltage configuration used in the voltage averaging drive, while FIG. 3 shows a voltage configuration used in the SA and MLA drives.
Referring to FIG. 2, in the voltage averaging drive, the voltages necessary for the scanning side are voltages V1, V2, V5, and V6, while the voltages necessary for the signal side are voltages V1, V3, V4, and V6. Referring to FIG. 3, in the SA drive, the scanning side needs voltages VCH, VM, and VCL, while the signal side needs voltages VSH and VSL. Further, taking the number of simultaneously selected scanning lines as three in the MLA drive, and by adding one dummy scanning electrode, there are two voltage levels used on the signal side, the voltages VSH and VSL, the same as those used in the SA drive.
When display on a full screen is mixed with display on only a portion of the screen in a simple matrix type liquid crystal display panel and the same driving method is used, a region where operation voltages differ is used due to differences in duty, and the design of an output portion of a driver IC becomes complex. Further, although the operating voltage range does not change when display is performed to only a portion of the screen without changing the duty, the electric power consumption does not decrease.